OBJECTIVE: The purpose of this study is to determine whether mothers of dizygotic twins, who have a higher level of gonadotrophins than other mothers, have cancer incidence or cancer mortality rates different from those of controls. The study will be based on a follow-up of the experience of a study group of 3,982 mothers who had male-female twins or other polyzygous births in Connecticut during the year 1925-1959; a control group of 3,982 women matched pairwise on the factors of year of childbirth, age, number of previous children, race and national origin; and a second control group of 3,982 women randomly selected from mothers of singletons. These women were identified during 1965-1967 in research on breast cancer among mothers of dizygotic twins. In addition, the Connecticut Twin Registry will be searched for male-female twins and other polyzygous births occurring during the period 1960-1974; we expect to identify 3,000-3,800 mothers of DZ twins from this search. The Vital Statistics Records will be searched for controls chosen in the ratio of 4:1, and matched on age, race and index year of birth. These women will be added to the study and the Connecticut Tumor Registry searched for the women in the study and control groups recorded since 1966 for cancers of all sites. The incidence of site-specific cancer will be determined. The rates observed will be compared with those expected in view of the rates among Connecticut residents of the age and sex.